In recent years, in order to protect global environmental, weight reduction of automobile bodies is demanded as a measure to decrease the amount of CO2 discharged from automobiles. For the purpose, increases in the strength of steel sheets for automobiles are required. This is because increase in the strength of steel sheet will allow decrease of the thickness of steel sheets for automobiles, thereby decreasing the weight of automotive bodies.
On the other hand, there are increased demands by society for safety of automobiles in collisions. Accordingly, there is a need for not only simply increasing the strength of steel sheet, but also developing a steel sheet having improved impact resistance upon collision during driving. Since each portion of a member for an automobile is subjected to deformation at a high strain rate of several 10 s to 103/s in collision, a high strength steel sheet having improved dynamic strength properties is required for automobile use.
As a high strength steel sheet having improved dynamic strength properties, there are known high-strength multi-phase structure steel sheets including a low-alloy TRIP steel sheet (strain induced transformation type high-strength steel sheet) which has a high static-dynamic difference (difference between static strength and dynamic strength) and a multi-phase structure steel sheet having a second phase primarily including martensite.
Regarding the low-alloy TRIP steel sheet, for example, Patent Document 1 discloses a strain induced transformation-type high-strength steel sheet having improved dynamic deformation properties and for absorbing automobile collision energy.
Prior art examples relating to the high-strength multi-phase structure steel sheet having a second phase primarily including martensite include the following Patent Documents.
Patent Document 2 discloses a high-strength steel sheet having improved balance of strength and ductility, and a static-dynamic difference of at least 170 MPa, and a method for its manufacture, wherein the steel sheet has a multi-phase structure comprising a ferrite phase and a hard second phase dispersed therein, in which the average grain diameter ds of nano crystal grains having a grain diameter of at most 1.2 μm and an average grain diameter dL of micro crystal grains having a grain diameter exceeding 1.2 μm in the ferrite phase satisfy a relationship of dL/ds≧3.
Patent Document 3 discloses a hot-rolled steel sheet having a high static-dynamic ratio, and a method for producing the same, wherein the steel sheet has a dual-phase structure of martensite having an average grain diameter of at most 3 μm and ferrite having an average grain diameter of at most than 5 μm.
Patent Document 4 discloses a cold-rolled steel sheet having improving impact absorbing properties, and a method for its manufacture, wherein the steel sheet has a dual-phase structure, which contains at least 75% of ferrite phase having an average grain diameter of at most 3.5 μm, the remainder being tempered martensite.
Patent Document 5 discloses a cold-rolled steel sheet having a static-dynamic difference of at least 60 MPa at a strain rate of 5×102 to 5×103/s, and a method for its manufacture, wherein the steel sheet is made to have a dual-phase structure of ferrite and martensite by pre-straining.
Patent Document 6 discloses a high-strength hot-rolled steel sheet having improved impact resistant properties, wherein the steel sheet has a dual-phase structure of at least 85% of bainite and a hard phase such as martensite.